1. Field of the Invention
The invention relates to charging and discharging control over a stationary electrical storage system.
2. Description of Related Art
Japanese Patent Application Publication No. 2004-185915 (JP 2004-185915 A) describes a replacement method in which part of a plurality of cells that constitute a battery pack is replaced with a replacement new cell. With this replacement method, the battery pack is allowed to be used without carrying out a post-replacement equalizing process by charging the new cell to a charged level lower than those of the cells other than the replacement-target unit cell.
However, charging efficiencies are different between the replaced new cell and each of the non-replaced old cells and between the old cells due to a difference in cell degradation, and the like. Therefore, there may be variations in charged level (SOC) among the cells that constitute the battery pack as a result of repetition of charging and discharging.
Particularly, different from a battery pack that outputs energy that is used to cause a vehicle to travel, a stationary storage battery has a wide usage range of the charged level (difference between an SOC upper limit value and an SOC lower limit value). If the usage range of the charged level is wide, the influence of the difference in charging efficiency further increases, and variations in the charged level tend to increase. Different from the vehicle battery that allows a large output at a time, in the stationary storage battery for which importance is placed on supplying electric power stably for a certain period, if variations in the charged level among the cells increase as a result of repetition of charging and discharging and the chargeable and dischargeable usage range reduces, it may not be able to exercise sufficient performance as the stationary storage battery.
In addition, in recent years, there is an effort to reuse a battery pack, mounted on a hybrid vehicle or an electric vehicle, as a stationary storage battery, and the used battery pack mostly has a difference in charging efficiency due to cell degradation and the like. Therefore, there is a possibility that the performance of the stationary storage battery cannot be sufficiently extracted and the used battery pack cannot be reused so as to fully use the battery pack.